The long-term goal of this project is to define the molecular mechanisms leading to tumor suppressor p53 accumulation and neuronal degeneration in the central nervous system. The application is based on novel findings obtained while conducting studies related to our previous NIH application. Activation of the p53 pathway has been strongly implicated in neuronal apoptosis in the mammalian central nervous system. In non-neural cells p53 abundance is tightly controlled by an inhibitory feedback loop involving the Mdm2 oncoprotein which binds to and targets p53 for ubiquitin-mediated degradation. However, the events responsible for p53 stabilization following DNA damage in neurons have not been delineated. Our preliminary studies demonstrate a novel mechanism of p53 accumulation due to down regulation of ubiquitin, as well as localization of p53 protein in the nucleolus of apoptotic neurons. The proposed studies will therefore use molecular and biochemical approaches to investigate the hypothesis that p53 stabilization is an essential component of certain types of neuronal degeneration mediated, in part, by impairment of nucleolar function. Specific Aim 1 will determine whether Mdm2 binding modulates p53 function, and will employ different approaches in animal and cell culture models to determine whether p53 accumulation is essential for neuronal cell death. Specific Aim 2 will investigate whether p53 regulates the ubiquitin-proteasome system. Specific Aim 3 will characterize the effect of p53 on nucleolar function in neuronal degeneration. Finally, Specific Aim 4 will determine whether neuronal death is prevented by restoring p53 degradation to vulnerable neurons. These studies should significantly add to our understanding of the molecular basis of certain types of neuronal cell death and the development of novel molecular and pharmacological interventions for neurodegenerative diseases.